1. Field of the Invention
This invention relates to an improved process for producing liquid hydrocarbons from coal. More particularly, this invention relates to a process for enhancing liquid hydrocarbon yields from solid coal by pretreating the coal to introduce aliphatic hydrocarbon radicals or acyl radicals (including carbon monoxide) into the coal structure. Still more particularly, in a preferred embodiment of this invention, the alkylated or acylated coal is subjected to liquefaction in the presence of hydrogen or a hydrogen donor solvent or both.
2. Discussion of the Prior Art
In recent years, the production of liquid hydrocarbons from non-petroleum sources has taken on added importance. Thus, with proven world petroleum reserves shrinking, other forms of energy have attracted attention. Perhaps, the greatest attention has been directed to coal, an abundant fossil fuel, particularly in the United States, which can be converted to liquid hydrocarbons at costs approaching current and projected costs for the refining of crude petroleum. Moreover, basic coal conversion technology exists and has been demonstrated on a variety of levels, e.g., pilot plant and full scale commercial (although highly expensive) plants. However, full development of existing conversion technology is only now under way.
Coking of coal with the attendant recovery of coal liquids is a long established process. Solvation of coal, with or without the addition of molecular hydrogen has also long been known as a feasible, if not economically attractive, process for producing coal liquids (see, for example, U.S. Pat. No. 1,342,790). The Pott-Broche Process (for example, U.S. Pat. No. 1,881,977) with modifications, was capable of producing gasoline from coal, albeit at then excessive costs. A number of process schemes for the liquefaction of coal using hydrogen donor solvents has also been suggested (for example, U.S. Pat. No. 3,617,513).
While there has been great emphasis on the conversion of coal to more useful liquid and gaseous products the investigation of the coal molecule, i.e., that which is to be converted, has often lagged and has been of relatively little importance. Nevertheless, an understanding of the material to be converted is elementary to the development of sound conversion technology. As a result, the chemistry of coal is now being actively pursued and while the structure of coal remains, for the most part, unresolved it is now generally believed that the coal molecule is not constructed on a diamond-like framework but rather it contains aromatic rings which are highly substituted (i.e., fused to other aromatics of hydroaromatics, or attached to alkyl, ether, hydroxyl, etc. groups). Additionally, it is now believed that coal exhibits secondary structural characteristics such as hydrogen bonding, interaromatic ring bonds, etc. which generate the three dimensional structure of coal. As a result of the condensed ring structure of coal, liquefaction processes have generally been limited by their ability to solvate exposed areas of the coal molecule. Thus, under normal liquefaction conditions, the secondary structural characteristics of the coal molecule are only partially, if at all, destroyed and a significant portion of the coal is not converted in the liquefaction process.
It has now been found that these secondary structural characteristics can be broken down, that is, the polymeric, three dimensional network structure of the coal is broken down by a non-destructive alteration of the coal molecule. Thus, the introduction of alkyl or acyl radicals into the coal molecule tends to increase the availability of reaction sites. In other, more simple, terms, more of the carbon in the coal is now susceptible to conversion at liquefaction conditions than would not have been available if the coal had not been alkylated or acylated.
Coal has been alkylated, primarily for investigation of the coal molecule. See, for example, C. Kroger, Forshungs Ber. Nordrhein-Westfalen No. 1488 (1965); H. W. Sternberg and C. L. Delle Donne, Fuel, 43, 172 (1974); H. W. Sternberg, C. L. Delle Donne, P. Pantages, E. C. Moroni and R. E. Markby, Fuel, 50, 432 (1971); J. D. Spencer and B. Linville, Bureau of Mines Energy Program, 1971, Bureau of Mines 1C8551, 1972; B. Linville and J. D. Spencer, Review of Bureau of Mines Energy Program, 1970, Bureau of Mines 1C8526, 1971; W. Hodek, and G. Kolling, Fuel, 52, 220 (1973) discussing the increase in extractability of bituminous coal by the related Friedel-Crafts acylation. Nevertheless, no prior reference has suggested that increased yields of liquid products via liquefaction can be obtained by first subjecting the coal to either alkylation or acylation. See, also, F. Meyer, Ph.D. Thesis, University of Munster, 1969; J. D. Spencer, Review of Bureau of Mines Coal Program, 1968, Bureau of Mines, 1C8416, 1969; J. D. Spencer, Review of Bureau of Mines Coal Program, 1969, Bureau of Mines, 1C8385, 1968, Sternberg, H. W. et al, The Electrochemical Reduction of a Low Volatile Bituminous Material, Fuel, 45 (6) 409-482 (1966). In "Coal Liquefaction by Alkylation Techniques," D. D. Denson and D. W. Buckhouse in a Stanford Research Institute paper dated 20 June 1975 prepared under a National Science Foundation grant, alkylation was utilized to enhance solvent refining but, again, no mention was made of enhancing liquid product yields by alkylation/acylation followed by convertiang the coal under liquefaction conditions.